Separation of plutonium ions from solution by adsorption on zirconium pyrophosphate



Unit es. P tenfO SEPARATION OF PLUTONIUM IONS FROM SOLU- TION BY ADSORPTION N ZIRCONIUM PYRO- PHOSPHATE Raymond W. Stoughton, Oak Ridge, Tenn., assignor to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Filed Dec. 19, 1944, Ser. No. 568,901

4 Claims. (Cl. 23-145) unless supported on a carrier such as rock wool or other inert material., Also, it is difficult to separatethe gelatinous zirconium phosphate from solutions which have contained the adsorbed substances.

It has been discovered that the gelatinous zirconium orthophosphate may be converted to a granular pyrophosphate form having adsorption properties similar to those of the gelatinous form but which is more conveniently used as an adsorbent. The granular zirconium phosphate is particularly suitable for adsorption of substances of the kind present in neutron irradiated uranium,

either by column or batch adsorption. Its high specific surface attraction for-plutonium makes it extremely useful in processes for separating plutonium from other substances by adsorption. The adsorbent used as herein contemplated may comprise zirconyl pyrophosphate .(ZrO) P O-; or the salt ZrP O 1 It is an object of the invention to provide an adsorption process utilizing granular zirconium phosphate. Further objects and advantages will appear from the following description.

The gelatinous zirconium phosphate is prepared by adding phosphoric acid to an aqueous solution of zirconium salt so that zirconium orthophosphate is thrown down as a gelatinous precipitate. The latter is converted to granular zirconium phosphate by dehydration, which may be accomplished by heating the gelatinous form under suitable conditions of time and temperature. After the dehydration has taken place, a zirconium phosphate is obtained as a white, amorphous mass comprising zirconium pyrophosphate which is prepared for use as an adsorbent by pulverizing the mass to the desired particle size. I

In preparing zirconium pyrophosphate, any soluble zirconium or zirconyl salt such as a nitrate, chloride, or acetate, is dissolved in water and precipitated with phosphoric acid as insoluble zirconium orthophosphate. An excess of phosphoric acid is used to insure precipitation of all the zirconium. To lessen formation of double salts and prevent the precipitation of impurities which may be present in the zirconium salt, the zirconium solution 2,970,035 Patented Jan. 31-, 1961 prior to the precipitation step may be acidified with mineral acids. Acidities of from 1 N to 10 N have been used satisfactorily. Where very pure zirconium orthophosphate is desired, the solution maybe acidified with nitric acid to between 4 N and 9 N acidity.

The zirconium orthophosphate precipitate is separated from the solution by filtration, centrifugation, or the like, and the precipitate dehydrated. Substantial dehydration may be effected by heating the precipitate under suitable conditions of time and temperature.

The temperature at which the zirconium orthophosphate is converted to zirconium pyrophosphate is necessarily above C. to substantially dehydrate the zirconium orthophosphate. Preferably, the zirconium phosphate is not heated to its melting point, as this would tend to destroy desirable physical characteristics obtained in converting the gelatinous form. It has been found that the ortho form may be converted to the pyro form by heating the former at a temperature of from 400 C. to 800 C. for fifteen hours, although as will appear more fully hereinafter this heating schedule may be departed from materially without seriously afiecting the adsorption efliciency of the resulting adsorbent.

The heat treatment converts the zirconium orthophosphate to a white, amorphous solid containing numerous and minute fissures and irregularities which increase, its surface area and aid its adsorptive properties. After cooling, the product is ground to the desired particle size for-use as an adsorbent. Particle sizes ranging from 50 to 200 mesh are particularly suitable for manyuses, and especially for the adsorption of Pu. The zirconium pyrophosphate obtained by the above method retains its granular form in water and in dilute acid solutions such as 1 M HNO3.

The high adsorption characteristics and granular form of zirconium pyrophosphate make it an excellent adsorbent for many purposes. It is contemplated that its use as an adsorbent will be similar to the uses of such adsorbents as silica gel, -zirconium orthophosphate, or similar adsorbents. Zirconium pyrophosphate is 'especiallysuitable for adsorbing substances of the kind present in neutron irradiated uranium in view of the specific surface attraction it has for such substances, and particularly for plutonium. Various processesinvolving these substances may be based on the use of zirconium pyrophosphate as an adsorbent; thus, plutonium, uranium, and fission products may be separated from each other and from foreign products by being adsorbed and subsequently desorbed into substantially separate fractions. The plutonium, when referred to in the following description, is in its reduced or phosphate insoluble state.

The zirconium pyrophosphate may be used in a batch process in which the finely divided adsorbent is placed in a solution containing the substances to be adsorbed and the solution agitated until adsorption is complete, whereupon the adsorbent carrying the adsorbed substances is separated from the solution by filtration, centrifugation, or-other suitable means. The adsorbed substances are removed from the adsorbent in any convenient manner such as washing with water, acid solutions, or other desorbing agents. Preferably, the zirconium pyrophosphate is used in column adsorption in which full advantage is taken of the granular nature of the adsorbent. In column adsorption, the granular adsorbent is placed in an elongated container and a solution comprising the substances to be adsorbed is flowed through the container. The adsorbates are removed by flowing wash solutions such as solutions of mineral acids through the container in sufficient quantity to obtain the degree of desorption desired.

In the following description, the isotope of element 93 having a mass of 239 is referred to as 93 and the isotope of element 94 having a mass of 239 is referred to as 94 Element 94 may also be designated as plutonium, symbol Pu. Reference herein to any of the elements is to be understood as denoting the. element ge: nerically, whether in its free state or in the form of a compound, unless indicated otherwise by the context.

Neutron irradiated uranium may be prepared by reacting uranium with, neutrons from any suitable neutron source, but preferably the neutron irradiated uranium is produced from a chain reaction of neutrons with uranium.

Neutron irradiation of uranium produces U which has a half-life of 23 minutes and by beta decay becomes 93 This element has a half-life of 2.3 days and by beta decay becomes 94 Neutron irradiated uranium contains 93 94 and a large number of radioactive fission products produced by reaction of neutrons on fissionable atoms, such as U which is present in uranium from natural sources. It also contains minor amounts of other products such as UX and UX Inasmuch as the 93 and 94 content of neutron irradiated uranium is produced in accordance with the neutron density of exposure times the time of exposure, and as the weight of radioactive fission products is proportional to the amounts of 93 and 94 formed, it is convenient to separate the desired elements when the combined amounts thereof are minute, such as, for ex{ ample, approximately 02% by weight of the irradiated uranium. 'By storing the neutron irradiated uranium for a suitable period 'of time, the 93 is converted almost entirely to 94 The fission products are present in. the neutron irradiated uranium generally to an'ex-tent of about 0.02% by weight. Because the fission products in general are highly radioactive, it is preferred that these materials be removed.

The fission products consist of a large number of;elementsv which may be classified into two groups; a light group with atomic numbers from 35 to 45; and a heavy group with atomic numbers from 51 to 60. The fission products. with which i am particularly concerned are those: having a half life of more than three days since. theyremain in the neutron irradiatedreaction mass in substantial quantities. at least one month after reaction. These. products are chiefly radioactive isotopes of-Sr, Y; Zr, Cb, and Ru of the group of-atomic numbers from 35 to 45; and Te, I, Xe, Cs, Ba, La,and Ce from the group of atomic numbers from 51 to 60, inclusive.

In carrying out an adsorption process using zirconium pyrophosphate to.separate substances of the kind present in neutron irradiated uranium, the neutron ir radiated uranium, containing minute amounts of plutonium and fission productssuch as, for example, approximately 0.01% byweight of plutonium and a similar amount of fission products, is dissolved in nitric acid to form a solution of uranyl nitrate hexahydrate, UO (NO .6H O, theuranyl nitrate hexahydratecontaining the dissolved plutonium and fission products. The zlrconium pyrophosphate, having a particle size of between 60to 100 mesh, is placed in the solution and the latteris shaken for sufiicienttime to permit adsorptron of thevarious substances. in the solution, and, partlcularly, the plutoniurr1 Where 5' grams of the zirconium pyrophosphate areused for each 100 cc. of'the uranyl nitrate hex'ahydrate solution, the solution may be shaken with the adsorbent for betweentwo andthree hours. The adsorbent carrying the various adsorbed substances may be removed from the solution by filtration, centrifugation, or other convenient means. The adsorbates; are removed from the zirconium pyrophosphate bywashing with water or other desorbing agents, such as solutions of mineral acids, that is, H 50 HNO and HCl, or the like.

In one embodiment of the invention, zirconium phosphate was precipitated from a 9 M HCl solution by an excess of phosphoric acid to form zirconium orthophosphate. The latter was washed with dilute HCl and air dried. It was then heated to 400 C. for four hours to form a white, amorphous solid, considered to be zirconium pyrophosphate. The latter was ground to be tween 60 and 100 mesh for use as; an adsorbent. One gram of the zirconium pyrophosphate was shaken with 20 cc. of a 10% solution of uranyl nitrate hexahydrate for 2 /2 hours and the adsorbent removed by centrifugation. Analysis of the substances adsorbed indicated that 79% of the plutonium present in the original solution was adsorbed on the zirconium pyrosphosphate and that 25% of the beta emitting fission products present in the original solution were adsorbed. The zirconium pyrophosphate was granular in nature and retained its granular structure during the adsorption process.

Zirconium pyrophosphate prepared as described above was variously treated to determine the effect of such treatments upon the adsorption characteristics of the treated pyrophosphates with respect to untreated pyrophosphate and with respect to silica gel. Thus, zirconiurn pyrophosphate of between 80 and 100 mesh was prepared as one gram samples and treated as follows: (A) untreated, (B) placed in water for 2 /2 hours, and (C) placed in a mixture of 0.1 M H PO and 1 M HNO for two hours, washed with very dilute HNO and finallywashed with water. These samples together with a one, gram sampleof, silica gel of between 80 and 100 mesh, wereeachshakenwith .20 cc. of. a 10% solution of; uranyl nitrate hexahydrate for 2 /2 hours and the particular sample of adsorbent removed by centrifugai he ti n har er stiss h. mp e ar given in Table I:

Table 1 Percent Percent Adsorbent Treatment of 942119 of fission adsorbed products Hyporsil (80-100 mesh) None 36 6 (A) Zirconium phosphate None 79 25 (80400 mesh). (B) Zirconium phosphate. Water 21 (SO- mesh). (G) Zirconium phosphate H;PO4-HNO; 79 22 (50-100 mesh) washed withwa r.

As shown in Table I all three samples of zirconium pyrophosphate have a high capacity for adsorption of plutonium when compared with a similar sample of silica gel. The above samples of the pyrophosphate retain their granular structure throughout the adsorption process.

It has been found that the adsorption capacity of the zirconium pyrophosphate is not greatly altered by rather great difierences in the temperature at which the zirconium orthophosphate is heated to convert it to the zirconium pyrophosphate. Thus, the zirconium pyrophosphate prepared by heating zirconium orthophosphate (a) at 400 C. for four hours and (b) at 800 C. for fifteen hours does not greatly alter its adsorption characteristics for plutonium.

Three samples of zirconium orthophosphate of /2 gram each were prepared by precipitation from 9 M HCl and air dried. One sample was heated at 400 C. for four hours, the secondsample was heated at 800 C. for fifteen hours and the third sample was heated at 800 C. for fifteen hours and pre-soaked in water for four hours before being used as an adsorbent. These samples were then shaken for two hourswith 100 cc, portions of a 10% solution of uranyl nitratehexahydrate. The. results obtained for adsorption and desorption for plutonium and for beta emitting fission products by these samples are aeersgoee Table II Eiutlon Subsequent Adsorption with cc. elution 6 M HNO, with 5 cc. Preparation of zirconium 8 M HNO,

phosphate Per- Per- Per- Per- Per- Percent cent cent cent cent cent Pu f.p. Pu f.p. Pu f.p.

Heated to 400 C. for 4 hr 87 19 31 14 15 4 Heated to 800 0. for 15 hr 83 12 56 8 12 2 Heated to 800 C. for 15 hr. and

pre-soaked in water for 4 hr. 78 16 46 8 12 2 The rate of adsorption of plutonium by zirconium pyrophosphate is retarded somewhat by the presence of uranyl ions. Zirconium orthophosphate Was prepared by being precipitated from 9 M HCl solution, washed with diluted HCl, and air dried. It was then heated at 600 C. for five hours to convert it to zirconium pyrophosphate. The latter was pulverized to between 60 and 100 mesh and pre-soaked in water for eighteen hours. This was then used to test the rate of adsorption of Pu from a 10% solution of uranyl nitrate hexahydrate in which 0.3 gram of the zirconium pyrophosphate were shaken with 10 cc. of the solution containing Pu Results are given in Table III, and these are to be compared with a nearly 100% complete adsorption of Pu in one minute from the solution containing no uranyl ions.

Table III Percent of p ns d. sorbed in Presence Time for Adsorption (minutes) It has been found that the completeness of adsorption of plutonium by zirconium pyrophosphate is somewhat lowered by the presence of the uranyl ion in the solution. Also, the equilibrium value for the plutonium desorbed by nitric acid or other mineral acid wash solutions is lower when the adsorption has taken place in the absence of uranyl ion than when uranium is present. Under the same conditions that a 40 to 50% adsorption of plutonium is obtained in thirty minutes in the presence of uranyl nitrate, the adsorption is nearly 100% complete in one minute in the absence of uranyl ion at a pH of from 2 to 0.

In the absence of uranyl ion in the original solution, however, the elution of the plutonium is more diilicult. Zirconium pyrophosphate prepared by heating zirconium orthophosphate at 600 C. for five hours was pulverized to between 60 and 100 mesh. 0.3 gram of the zirconium pyrophosphate was shaken for two hours with 100 cc. of a nitric acid solution of plutonium 238 which had a pH of 2.4 and with no uranyl ion present. 100% of the Pu was adsorbed by the zirconium pyrophosphate. To ascertain various conditions of desorbing or elution, the 0.3 gram of the zirconium pyrophosphate carrying the adsorbed Pu was agitated with '10 cc. of the various eluant solutions. The results of this test are given in Table IV. A constant shaking on a rotor in the 25 C. experiments and frequent stirring in the 93 C. test was helpful. A certain amount of disintegration of the ad sorbent occurred in these tests, especially in the solutions where v3 M H PO, was used.

Table IV Temper- Percent Eluant used ature of 94 elution eluted 6 M HNO 93 19 6 M HNO -1 mgm. Zl- 93 19 8 M HNO: 93 24 3 M HNOr-S M H P0 93 53 3 M HBPOi 25 25 1 M HzPor 25 3 1 M H3PO46 M HNOQ-.- 25 18 3 M HaPO4-3 M HNOe.-- 25 40 The description thus far has been concerned with a batch adsorption process using zirconium pyrophosphate, for purposes of illustration. However, it is contemplated that the zirconium pyrophosphate may be even more effectively used in a column adsorption process because of the advantages obtained from its granular structure as well as its adsorption characteristics in general. In column adsorption, the zirconium pyrophosphate of between 50 and 200 mesh or other suitable particle size is placed in a container, such as a cylindrical column, and the solution containing the substances to be adsorbed is flowed through the column at a sufiiciently slow rate to permit the adsorption of the particular substances desired. The quantity and kind of substances adsorbed may be controlled to some degree by the rate of passage of the solution through the column or by the particle size of the adsorbent. After the adsorption is complete, the various adsorbates may be removed by flowing desorbing agents such as mineral acid solutions through the column.

Where two or more substances are adsorbed by a process of column absorption, there is generally a tendency for such adsorbates to be positioned in the column according to their respective adsorption affinities for the adsorbent. Thus, where plutonium, fission products, and uranium are adsorbed by passing a solution containing them downwardly through a column of zirconium pyrophosphate, a layer effect will result with the plutonium (which has the greatest adsorption affinity for the particular adsorbent) being adsorbed in the upper portion of the column, the fission products below the plutonium, and the uranium below the fission products. While such layers will usually be well defined, in certain instances there may be some overlapping depending upon various conditions of the process, such as the flow rate of the solution, the size and shape of the container, and the amount of adsorbent used.

The above process is known as chromatographic adsorption, and is useful in separating adsorbed substances from each other and from foreign products inasmuch as "the process permits desorption of the adsorbates according to their respective adsorption affinities, that is, the adsorbate having the lowest adsorption affinity is removed by the wash solution from the column first, the adsorbate having the next lowest adsorption affinity is removed secondly, and so on. As each adsorbate is removed, it may be collected as a substantially separate fraction. Mineral acid solutions of various compositions and strengths can be used to desorb one or more of the adsorbates.

In view of the specific adsorption characteristics of the zirconium pyrophosphate for plutonium, it is particularly effective in separating plutonium from other substances such as those present in neutron irradiated uranium.

,9. QiQ3 '3" above and then collected as a substantially separate fraction.

The above detailed description isgiven for purposes of illustration and the invention is to be limited only by the scope of the following claims.

' What is claimed is:

1. A method of separating plutonium from other sub-t stances present in. a solution containing ionsof'a. compound of plutonium in which the plutonium is in its re- I duc'ed, phosphate-insoluble state "and ions of other substances which comprises contactingtthe solution with granular zirconium pyrophosphate whereby said plutonium ions are adsorbed leaving the ions. oi the other substances in solution.

'2. A method of separating plutonium from other substances in a solution containing ions of a compound of plutonium in which the plutonium is in its reduced, phosphateeinsoluble state which comprises adsorbing said plutonium and other substances on granular zirconium pyrophosphate, and desorbingsaid other substances while saidplutonium having avalent state of not greater than +4 remains adsorbed.

3. The process of obtaining plutonium in a more concentrated statefrom neutron irradiated uranium containing ions of a compound of plutonium in which the plutonium ions are in its reduced, phosphate insoluble state and ions of compounds of uranium and fission 7 phosphate, whereby said plutonium ions are adsorbed, from solution leaving ions of compounds of said fission,

products and uranium in solution, and thenremoving the adsorbed, plutonium and the zirconium phosphate from' solution.

4. The process of separating plutonium fro m an aqueousv solution containing ions of a compound of plutonium' in which the plutonium ions are in its reduced,

phosphatednsoluble state, the step of contacting said solution with granular zirconium pyrophosphate whereby said plutoniumionsare-adsorbed from the aqueous soluproducts which comprises forming a solution containing ions of elements present in neutron irradiated uranium, contacting said solution with granular zirconium pyrotion, and then Separating the adsorbed plutonium and zirconium phosphate from the remaining solution.

" OTHER REFERENCES I I I Mellor: Inorganic Chem. vol. 7 (pages 163 and 164) 'Longmans, Green 8: Co. (NY.), 1927. Clopy in Division 

1. A METHOD OF SEPARATING PLUTONIUM FROM OTHER SUBSTANCES PRESENT IN A SOLUTION CONTAINING IONS OF A COMPOUND OF PLUTONIUM IN WHICH THE PLUTONIUM IS IN ITS REDUCED, PHOSPHATE-INSOLUBLE STATE AND IONS OF OTHER SUBSTANCES WHICH COMPRISES CONTACTING THE SOLUTION WITH GRANULAR ZIRCONIUM PYROPHOSPHATE WHEREBY SAID PLUTONIUM IONS ARE ABSORBED LEAVING THE IONS OF THE OTHER SUBSTANCES IN SOLUTION. 